1. Field of the Invention
This invention concerns a method of manufacturing a semiconductor device. More particularly, this invention relates to a method of forming an improved trench region for the purpose of isolation of semiconductor elements formed in a semiconductor substrate.
2. Description of the Prior Art
Conventionally, a trench method using trenches filled with a dielectric material and a polycrystalline silicon layer is used for the separation of semiconductor elements. FIG. 1 shows an example of a conventional trench method. At first, an insulating layer 3 of sufficient thickness to serve as a mask layer for a following etching process is formed on the surface of the epitaxial growth layer 2 of P type (or N type) formed on the semiconductor substrate 1 of N type (or P type). The insulating layer 3, e.g., a silicon oxide layer, is selectively removed by conventional photoetching techniques to expose the surface of the layer 2 where a trench region is to be formed, as shown in FIG. 1A. Next, an etching process is carried out to form trench regions 5 using the insulating layer 3 as the mask. The depth of the trench regions 5 is chosen approximately equal to the depth of an isolation region to be formed. In this condition, an ion-implantation process, using dopant of the same conductivity type to the substrate 1, is carried out to form a channel stopper region (not illustrated) beneath the trench region to reduce a parasitic MOS effect. Next, an oxidation treatment is carried out to form an insulating layer 4, e.g., a silicon oxide layer, for the purpose of protection and partial filling of the trench regions 5. Next, a polycrystalline layer is formed to fill and cover the trench regions 5, completely. Next, an etching process is carried out to remove any excess polycrystalline layer above the top of the trench regions 5(FIG.1C). Then, an oxidation treatment is carried out to form an insulating layer 7 on the top surface of the polycrystalline layer 6 to get a flat surface as shown in FIG.1D. In this way, insulated island regions 201, 202 and 203 are achieved. In this conventional process, at the formation of the thick insulation film 4, e.g., between 8,000 .ANG. and 10,000 .ANG., great stress is applied to the corner of the trench region 5. In particular, since the corners are sharp, stress concentration occurs at the corners. This stress causes crystal defects 19 in the island regions 201, 202 and the substrate 1, as shown in FIG. 1E. To reduce the stress during the formation of the thick insulating layer, another process using a thin insulating film has been used. FIG. 2 shows an example using a thin insulating film 12, e.g., less than 2000 .ANG.. FIG. 2A shows a state after the formation of insulated island regions 11 and 14. However, in the case where a thick insulating layer 16, such as a field oxide layer, is successively formed, wedge-shaped insulating regions are formed at the corners of the island regions 11 and 14, as illustrated by numeral 17(FIG. 2B). Thus, stress is applied to the corners of the epitaxial layers 11 and 14, and crystal defects also are created. FIG. 3 shows an improved conventional process to reduce the stress concentration at the corners of the trench region by rounding the corners thereof. After forming the trench region in a semiconductor substrate 20 as illustrated in FIG. 3A, an insulating layer 21 is formed, as illustrated in FIG. 3B. Next, the insulating layer 21 is removed, as illustrated in FIG. 3C. As the insulating layer 21 is formed by the reaction of oxygen diffused into the substrate 20 with the material, e.g., silicon, of the substrate 20, the boundary between the insulating layer 21 and the substrate 20 is determined by the profile of the diffused oxygen. As the constant density line of the diffused oxygen at the corners is round, the formation of a rounded corner would be expected. However, at the formation of the insulating layer 21, stress is also applied to the substrate. The more the stress is applied, slower the growth rate of the insulating layer becomes. As the stress concentration occurs at the corners, the growth rate of the insulating layer at the edge of the corners is slower than that at other portions. As a result, a sharp portion is formed at the corner of the trench region, as illustrated in FIG. 3D. This process is also insufficient to round the corner. In the case where a trench capacitor (not illustrated) is formed in the trench region, the sharp portion causes a concentration of the electric field.